SIE[mu]
can be used as input for -dimensional with -dimensional Lorentz index and is transformed into PauliSigma[LorentzIndex[mu,D-4],D-4]
by FeynCalcInternal.
Overview, PauliSigma, SI.
[\[Mu]] SIE
[\[Mu], \[Nu]] - SIE[\[Nu], \[Mu]] SIE
StandardForm[FCI[SIE[\[Mu]]]]
(*PauliSigma[LorentzIndex[\[Mu], -4 + D], -4 + D]*)
[\[Mu], \[Nu], \[Rho], \[Sigma]] SIE
[\[Mu], \[Nu], \[Rho], \[Sigma]] // StandardForm
SIE
(*SIE[\[Mu]] . SIE[\[Nu]] . SIE[\[Rho]] . SIE[\[Sigma]]*)
[\[Alpha]] . (SISE[p] + m) . SIE[\[Beta]] SIE